1. Filed of the Invention
The present invention relates to a detachably attachable sheet cassette for accommodating sheets, and, more particularly, to a sheet feeding device for feeding sheets or to a sheet cassette used in an image forming apparatus such as a printer, a copying machine, a facsimile, and the like for forming an image on a sheet.
2. Description of the Related Art
Heretofore, image forming apparatuses are provided with a sheet cassette in detachably attachable manner in which sheets to be supplied for image formation are accommodated. As disclosed in Japanese Patent Application Laid-Open Publication No. 8-217260, the sheet cassette used by being mounted on the image forming apparatuses in detachably attachable manner includes a sheet cassette that can accommodate sheets of different sizes.
FIG. 11 exemplifies a sheet cassette 50 that can selectively accommodate A3 size sheets S1 and 11×17 inch size sheets S2. The sheet cassette 50 shown in FIG. 11 includes a pair of sheet width regulating plates 51, 52 for regulating the positions of the accommodated sheets S1 or S2 in a width direction. The sheet width regulating plates 51, 52 are disposed movably in the sheet width direction so that they are aligned with the side edges of accommodated sheets S1, S2. Further, the sheet cassette 50 includes a back guide 23 for regulating the positions of the rear edges of the sheets S1 or S2. The back guide 23 is disposed so as to move in a sheet feeding direction so that it is aligned with the rear edges of the accommodated sheets S1, S2.
Further, the sheet cassette 50 shown in FIG. 11 is arranged such that it can be drawn from and accommodated in an image forming apparatus body in a direction orthogonal to a sheet conveyance direction. The sheet cassette 50 arranged as described above is provided with ribs 53, 54 at positions corresponding to the A3 size sheets S1 and the 11×17 inch size sheets S2, respectively to prevent the rotation (positional non-alignment) of the rear edges of the sheets due to a shock when the sheet cassette is inserted.
Recently, image forming apparatuses capable of forming a color image become widespread, the size of sheets used by users becomes diversified. Specifically, a demand for sheet sizes wider than, for example, the A3 size described above is increased as the sizes of sheets to be used by the users, and thus the sheet cassette is required to support the sheets of these sizes. Note that, as the size wider than the A3 size, there is a sheet size called a full bleed sheet, for example, RA3 (305 mm×430 mm), 12×18 inches (304.8 mm×431.8 mm), SRA3 (320 mm×450 mm) and the like.
When a sheet having a size wider than the A3 size is supported, the number of ribs for preventing the rotation of the sheet described above must be increased to the number of the sizes of the sheets supported thereby. However, since sheets have different lengths in the conveyance direction and in the width direction depending on sizes, even if the sizes of the sheets to be supported are increased, the number of the rotation prevention ribs cannot be increased accordingly. Therefore, a problem arises in that even if the number of the sizes that can be accommodated in the sheet cassette is increased, sheets may be rotated when the sheet cassette is inserted depending on the sizes of the accommodated sheets. Then, when a sheet is fed out in this state, since skew feeding of the sheet occurs, an image is deteriorated by jam (clogging of sheet) and the non-alignment of the sheet from an image position.